1. Field of Invention
The present invention relates to a driving circuit. More particularly, the present invention relates to a driving circuit to drive an output stage comprising a high side NMOS and a low side NMOS.
2. Description of Related Art
An output stage with high efficiency and small area is an important issue of a power amplifier circuit. The conventional design of the output stage comprises a PMOS and an NMOS. However, in the latest technology, the output stage comprises two NMOS as depicted in FIG. 1. The two NMOS output stage 1 comprises a high-side NMOS 10 and a low-side NMOS 12. The high-side NMOS comprises a drain, a gate and a source, wherein the drain is connected to a first voltage (PVDD), the gate is connected to a high-side driver 11 and the source is connected to an output terminal (OUT). The low-side NMOS 12 comprises a drain, a gate and a source, wherein the drain is connected to a second voltage (PVSS), the gate is connected to a low-side driver 13 and the source is connected to the output terminal (OUT). Although such a design provides smaller area and greater efficiency, it brings new problems as well. One of the problems is that the maximum value of Vgs and Vds of the NMOS are not symmetric, wherein the Vgs can't be a high value. Therefore, if the high side driver 11 of the high side NMOS 10 of the output stage 1 has an inappropriate design, the high side NMOS 10 would be broken since the output voltage of the source side should be much higher than the input voltage of the gate side to provide an amplification function.
Accordingly, what is needed is a driving circuit to drive an output stage and provide a mechanism to protect the high side NMOS to overcome the above issues. The present invention addresses such a need.